The present invention relates to a workpiece holder for polishing which is used for precision polishing of surfaces of workpieces such as semiconductor wafers, method for producing the holder, method for polishing a workpiece, and polishing apparatus.
In the conventional polishing operation, a plate composed of a rigid material such as glass, metal and ceramics is used as a workpiece holder, and a workpiece is held by adhering it on the surface of the holder with an adhesive such as wax, or by vacuum adsorption using a workpiece holder surface composed of a gas-permeable porous material or a workpiece holder surface provided with many perforated holes. However, if a workpiece is held directly on a workpiece holder surface composed of metal, ceramics or the like, cracks or stains may be generated on the back surface of the workpiece. Therefore, in order to prevent such a phenomenon, for example, it has been proposed to coat the workpiece holder surface with an extremely thin resin film having a thickness of several tens micrometers and composed of a material such as those known with the trade names of Teflon, Nylon etc. or polyvinyl chloride as disclosed in Japanese Patent Application Laid-open (Kokai) No. 58-180026, or there has been proposed a vacuum plate comprising an acrylic resin plate provided with holes for vacuum adsorption and adhered to a workpiece holder surface, wherein the resin surface is subjected to a polishing process afterward, as disclosed in Japanese Patent Application Laid-open (Kokai) No. 4-206930.
Further, there has been a problem in a polishing process that, even if the workpiece holder has a flat finished surface, a flat workpiece cannot necessarily be obtained after the processing because of the use of a soft polishing pad for polishing. That is, surface conditions of the polishing pad may be gradually changed by creep deformation of the polishing pad and so forth. There has also been proposed a method for producing an adsorption plate fitted to the creep deformation of the polishing pad as a part of the workpiece holder surface by coating the workpiece holder surface with a thin resin layer and polishing the surface, as disclosed in the aforementioned Japanese Patent Application Laid-open (Kokai) No. 58-180026. However, this method suffers from a problem that, because the resin layer is thin, it is worn out within a short period of time, and thus it becomes unsuitable for use.
Furthermore, when resin is coated on a workpiece holder surface with coating techniques such as spraying, there may be also arisen a problem that the holes provided in the workpiece holder surface for the vacuum adsorption are blocked with the resin. Therefore, Japanese Patent Application Laid-open (Kokai) No. 4-206930 proposed a method comprising adhering an acrylic resin plate provided with holes and having a thickness of 3-50 mm on a workpiece holder surface, and carrying out polishing directly on a polishing turn table of polishing apparatus. However, in this method, there is arisen a problem that, because the acrylic resin plate is adhered on the workpiece holder surface, the holes may be blocked with an adhesive or the adsorption plate may be exfoliated during the processing.
Further, according to findings obtained by the inventors of the present invention, when a resin plate having a thickness of 5 mm or more is used, the rigidity of the workpiece holder surface is degraded, and hence there is caused a problem of degraded polishing precision for workpieces. On the other hand, when a resin plate having a thickness of 3 mm or less is used, there is arisen a problem that unevenness of adhesive between the resin plate and the workpiece holder body is transferred to the surface of workpiece.
Furthermore, the workpiece holder body, which is usually made of metal, glass or the like, shows slightly different heat generation profiles for polishing of a workpiece holder surface adhered with a resin plate or the like on an polishing apparatus for forming its surface shape, and the subsequent polishing of a workpiece held on the surface. Thus, there is also arisen a problem that the workpiece holder body itself is deformed by heat to cause deformation of the surface shape of the workpiece holder produced with much troubles, which makes the shape of the workpiece after the polishing irregular.
Therefore, the present invention was accomplished in view of the aforementioned problems, and its main object is to improve the material of holder body of a workpiece holder for polishing that holds a workpiece by vacuum adsorption, and the material of resin film for coating a workpiece holding surface of the holder, and develop a method for coating the surface with a resin, which does not cause blocking of perforated holes of the holder body with the resin during the surface coating process with the resin, thereby providing a workpiece holder for polishing having a workpiece holding surface of high precision, and a method for producing it.
In order to achieve the aforementioned object, the present invention provides a workpiece holder for polishing having a workpiece holder body provided with multiple perforated holes for holding a workpiece by vacuum adsorption, wherein a holding surface of the holder body is coated with a coating film formed by applying a thermosetting resin on the holding surface and curing it with heating, and a surface of the coating film is polished.
By using a workpiece holding surface of the holder body coated with a coating film which is formed by applying a thermosetting resin on the surface and curing it with heating, and has a polished surface as described above, the unevenness of the adhesion can be eliminated, which has been generated in the previous cases where a resin plate is adhered to a workpiece holding surface of a holder body with an adhesive. As a result, the problems, i.e., the transfer of uneven adhesion between the resin plate and the workpiece holder during the polishing and so forth, are eliminated, and thus it becomes possible to perform high precision polishing of workpieces with the desired high flatness and no waviness thanks to the high precision holding surface of the resin coating film itself.
In this case, the thermosetting resin can be one selected from epoxy resins, unsaturated polyester resins, urethane resins and phenol resins.
By using a material selected from those mentioned above as the thermosetting resin, physical properties of the coating film after the heat curing required for the present invention, including hardness, mechanical strength, coefficient of linear thermal expansion and so forth, can be satisfied.
Further, in this case, viscosity of the thermosetting resin is preferably 10,000 cps or higher, and it is preferred that it is degassed by kneading under vacuum.
By using a thermosetting resin of high viscosity as described above, it becomes possible to easily prevent the blocking of perforated holes of a small diameter for vacuum adsorption provided in the resin coating film after the heat curing.
Moreover, if air bubbles remain inside the resin coating film or on the surface thereof after heat curing, they are transferred to the surface of workpiece during the workpiece polishing. Therefore, the thermosetting resin of high viscosity is preferably kneaded under sufficient vacuum for degassing before it is applied to the holding surface of the workpiece holder so that air in the resin should be removed.
Further, in this case, the resin coating film covering the holding surface of the holder body preferably has a thickness of 0.5 to 3 mm, and the surface of the resin coating film is preferably subjected to correction by lapping and then correction by polishing on a polishing turn table of a polishing apparatus.
When a resin coating film having a thickness of 3 mm or less is used as defined above, the rigidity of the workpiece holder body is not degraded, and thus polishing of workpieces can be realized with higher precision. And high flatness can be obtained with a thickness of 0.5 mm or more. Further, by subjecting the surface of the heat-cured resin coating film first to surface correction by lapping and then correction by polishing on a polishing turn table of a polishing apparatus, a holding surface of holder body of higher precision can be formed, and use of this workpiece holder enables workpiece polishing providing high flatness.
Further, the diameter of the perforated holes of the workpiece holder body can be selected from the range of 0.4 to 0.8 mm, and coefficient of linear thermal expansion of the material of the workpiece holder body is desirably 1xc3x9710xe2x88x925/xc2x0 C. or less. In addition, the material of the workpiece holder body is preferably silicon carbide (SiC).
If a diameter of the perforated holes of 0.4 mm or larger is used as described above, the holes become unlikely to be blocked with the resin when the resin coating film is formed on the holding surface of the holder body. On the other hand, a diameter of 0.8 mm or less can prevent the transfer of the traces of the holes due to unduly large holes during the polishing of a workpiece.
Further, by forming the workpiece holder with a material of a low coefficient of thermal expansion, difference of the thermal deformation degrees of the workpiece holder body during the polishing of the holding surface of the workpiece holder body on a polishing turn table of a polishing apparatus and the polishing of a workpiece can be reduced. Therefore, the high precision holding surface shape of the workpiece holder can be maintained, and thus workpiece polishing providing high flatness can be realized.
As a material of a particularly low coefficient of thermal expansion, high rigidity and corrosion resistance that makes the material unlikely to be corroded by polishing solution and so forth, silicon carbide is preferred.
The present invention further provides a method for producing a workpiece holder for polishing having a workpiece holder body provided with multiple perforated holes for holding a workpiece by vacuum adsorption and a holding surface coated with a resin, which comprises coating a holding surface of the workpiece holder body with a thermosetting resin, curing the thermosetting resin by heating to form a resin coating film while passing a gas through the perforated holes from a back face of the workpiece holder body, and subjecting a surface of the resin coating film to correction by lapping and then correction by polishing on a polishing turn table of a polishing apparatus.
According to this production method, the thermosetting resin can be cured without blocking the multiple holes of a small diameter provided in the workpiece holder body, and the thermosetting resin coating film can be formed without causing problems such as unevenness of adhesion on the holding surface of the workpiece holder body.
Moreover, by subjecting the surface of the heat-cured resin coating film to surface correction by lapping and then-correction by polishing on a polishing turn table of a polishing apparatus, the holding surface shape of the holder body can be formed with high precision, and thus it becomes possible to perform polishing of workpieces with high flatness.
In this case, when the holding surface of the workpiece holder body is coated with a thermosetting resin in the aforementioned production method, the thermosetting resin in the vicinity of the perforated holes can be preliminarily cured while passing a heated gas through the perforated holes from a back face of the workpiece holder body, and then the remaining resin can be cured by heating. Further, temperature of the gas can be set at a temperature the same as or higher than the heat curing temperature of the aforementioned thermosetting resin.
According to this method, since the resin in the vicinity of the perforated holes is cured first, the blocking of the perforated holes can more surely be prevented. Further, while the gas can be set at a temperature the same as or higher than the heat curing temperature of the aforementioned thermosetting resin, if the gas temperature is too high, curing rate in the vicinity of the perforated holes becomes high, and thus hardness of the resin may be different between the vicinity of the perforated holes and the other areas. This may make it impossible to perform the polishing with high precision. Further, heat curing reaction rate of the resin becomes a more potent rate-determining factor rather than the viscosity decreasing rate by heating. Therefore, in order to prevent the blocking of the perforated holes, the temperature is preferably substantially the same as the heat curing temperature of the thermosetting resin.
The present invention provides a method for polishing a workpiece, which comprises holding a workpiece on its back face by vacuum adsorption on the resin coating film surface used as a workpiece holding surface of the aforementioned workpiece holder body for polishing, and contacting the workpiece with a polishing pad to polish a surface of the workpiece.
According to the present invention, a holding surface shape of a workpiece polishing holder body can be formed with higher precision. Therefore, by using this workpiece holder for polishing, it becomes possible to perform high precision polishing of workpieces with the desired high flatness and no waviness.
The present invention further provides an apparatus for polishing a workpiece comprising a turning table provided with a polishing pad by adhesion, means for feeding a polishing agent to a surface of the polishing pad, and a workpiece holder for polishing which forcibly brings a workpiece into contact with a surface of the polishing pad with pressure, wherein the workpiece holder for polishing is the aforementioned workpiece holder for polishing.
By using a polishing apparatus comprising a workpiece holder for polishing wherein a thermosetting resin coating film is formed on a holding surface of a workpiece holder body as described above, a workpiece can be polished so that the desired high flatness should be obtained with no waviness. When the workpiece is a semiconductor wafer, in particular, the defocus failure in lithographic light exposure in the production process of highly integrated devices can be reduced, and thus yield and productivity of highly integrated devices can be improved.
As explained above, a workpiece holder for polishing having a workpiece holding surface of high precision can be provided by the present invention. Therefore, a workpiece having a surface with high flatness and no waviness can be produced by using this holder for polishing. In particular, when the workpiece is a semiconductor wafer which is polished by using the workpiece holder for polishing of the present invention, the defocus failure in lithographic light exposure in the production process of highly integrated devices can be reduced, and yield of highly integrated devices can be improved.